The invention is based on so-called open thermal switches, which are e.g. installed in electric heaters, such as electric radiators, hair dryers, etc. and cause a disconnection when an excess temperature occurs. It is also desirable with such switches that if the device has a malfunction, a "clocking", an opening and closing of the device, is prevented. Such a "clocking" can occur if, due to a malfunction of the device, there is an excess temperature, which opens the temperature switch, whose active switching element is generally a bimetallic element, so that there is no further current flow and a cooling can occur, through which the switch closes again and then, as a result of the malfunction of the device, an excess temperature again occurs and the switch reopens. To prevent this, a resistor is positioned in electrical parallel to the switch. In this connection thick-film resistors such as PTC resistors are known which are connected to the switch, e.g. by clips, shrink sleeves and the like.
DE-A-3,231,136 discloses a bimetallic switch with an insulating support part, in which are fixed connecting elements, in that e.g. the plastic carrier material is injection moulded around the connecting elements. From the connecting elements are bent tongues which carry the fixed and movable contact. Below the support part is provided a thick-film resistor in the form of a flat, conductively coated ceramic plate, which has rigid connection lugs, which are soldered to the connections lugs of the switch.
In addition to the aforementioned elements, the switch obviously also has a spring element carrying the movable contact stud, as well as a bimetallic disk for causing the switching process. The process has a complicated construction and is expensive to manufacture. Assembly operations must be carried out manually. It is also undesirable that the switch is made from numerous different materials, namely apart from the sheet metal of the connecting lugs, the switch is also made from plastic, ceramic and a conductive thick-film. It is also disadvantageous that the bimetallic element is fixed both at its ends and centrally, so that the switching action is reduced and it can also be exposed to permanent forces, which can modify the switching point.
DE-A-3,539,425 discloses a thermal switch. In this switch between prongs bent out from connecting lugs is positioned a thin, insulating ceramic plate which is soldered thereto. The ceramic plate is metallized and to one connecting lug is fixed a spring element of the movable contact. The other connecting lug is provided with the fixed contact stud. In the center of the spring element is fixed a rod or bolt, e.g. by means of a rivet traversing the bimetallic disk, which rests on the ceramic part. The ceramic part is also metallized, so that the metal coating forms the high-resistance, parallel resistor for keeping open the switch following opening by the bimetallic element. In practice, this switch has led to problems. It has a complicated construction, because the insulating ceramic plate must also be provided with a conductive coating. The opening through which the rod centering the bimetallic disk projects weakens the already thin ceramic plate, so that it can easily be broken. The switching path of the bimetallic disk is limited by the central fixing. The spring action and therefore the switching action of the spring carrying the movable contact is impaired by the rivet located thereon.